Electrosynthesis is a method for production of chemical reaction(s) that is electrically driven by passage of an electric current, typically direct current (DC), through, for example, a liquid, such as an electrolyte, between an anodic electrode (anode) and a cathodic electrode (cathode). An electrochemical cell is used for electrochemical reactions and comprises an anode and cathode which are in intimate contact with the electrolyte. The current is generated from an external power source and is passed between the two electrodes. Each electrode typically comprises about half of the electrochemical cell. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, DC is passed between the two electrodes in an aqueous electrolyte to split water, the reactant, into the component product gases, hydrogen and oxygen, which evolve at the surfaces of the respective electrodes. Water electrolysers have typically relied on cell separator membranes or diaphragms combined with pressure control systems to control the pressure between the two halves of an electrolysis cell to ensure that the gases produced in the electrolytic reaction are kept separate and do not mix.
As used herein, the terms “cell”, “electrolysis cell”, “electrochemical cell” and equivalent variations thereof refer to a structure comprising an anodic electrolysis chamber and a cathodic electrolysis chamber. Selectively permeable membranes or diaphragms (both referred to herein after as “membranes”) are disposed within the cell to prevent the gases produced at each electrode from intermixing within the electrolysis cell. An “electrolysis chamber” comprises one electrode and is separated from the other electrolysis chamber of the cell by the membrane. The electrolysis chamber is referred to as an anodic electrolysis chamber or a cathodic electrolysis chamber depending on whether the electrode is an anode or a cathode, respectively. In each electrolysis chamber, electrodes are typically mounted in close contact with the membrane. This can be accomplished, for example, by pressing the membrane between the electrodes. Membranes that are particularly suited to this purpose are described in EP-A-0 232 923 and U.S. Pat. No. 6,554,978, both of which are hereby incorporated by reference.
Multiple cells may be connected either in series or in parallel to form what are commonly called electrolyser cell “stacks”. Theoretically, there is no limit to how many cells may be used to form a cell stack. The term “electrolyser” or “electrolyser module” refers to the combination of an electrolyser cell stack and such peripheral components as degassing chambers and the necessary piping to connect the operative parts. The term “electrolyser system” refers to an electrolyser module and any equipment used in combination therewith, such as power supply equipment, water purification and supply equipment, and may also include gas conditioning and compressing equipment, electricity regenerator equipment, and equipment for storage and subsequent dispensing of the gas.
In the operation of a cell stack during the electrolysis of aqueous electrolyte, the anode serves to generate oxygen gas whereas the cathode serves to generate hydrogen gas. The two gases are kept separate and distinct by the membrane. In some types of electrolyser modules currently used, the flow of gases and electrolytes within the stack may be conducted via circulation gasket assemblies which also act to seal the structure of one electrolysis chamber to another electrolysis chamber and to prevent leakage of electrolyte and gas from the structure.
In some electrolyser modules currently used, an end box is situated at both ends of the stack. The end boxes serve several functions including providing a return channel for electrolyte flowing out from the top of the cell. They may also provide a location for components used for controlling the electrolyte level, for example, liquid level sensors, and temperature regulators, for example, heaters, coolers or heat exchangers. In addition, with appropriate sensors in the end boxes, individual cell stack electrolyte and gas purity may be monitored. Also, while most of the electrolyte is recirculated through the electrolyser, an electrolyte stream may be taken from each end box to provide external level control, electrolyte density, temperature, cell pressure and gas purity control and monitoring. This stream would be returned to either the same end box or mixed with other similar streams and returned to the end boxes. Alternatively, probes may be inserted into the end boxes to control these parameters.
As a gas is produced in the anodic electrolysis chamber or cathodic electrolysis chamber of the cell, it is mixed with the liquid of the cell to form a gas-liquid mixture. The gas will rise to the top of the cell but it requires time and space to become effectively separated from the liquid (through, for example, natural phase separation) and subsequently released. The end box as described above is sometimes used as an area to allow gas-liquid separation. Another typically used mechanism to allow the gas-liquid separation to occur is to utilize degassing chambers which are located externally to the cell stack. Each gas generated in the cells may enter a degassing chamber through a series of pipes or other connections attached to the cells. The degassing chambers may be located above the cells and benefit from the lower density of the gas and the gas-liquid mixture (relative to the liquid alone) which causes them to rise up to the degassing chambers. This location of the degassing chambers relative to the cells allows the degassed liquid to flow back down to the cells with the aid of gravity. Alternatively, pumps are used to pump the gas-liquid mixtures to the appropriate degassing chambers. The extra equipment required in these scenarios has the drawback that it is costly and it increases the space requirement for the electrolyser module. In addition, the extra pieces required to connect the degassing chambers to the cells provide opportunities for leakage, resulting in costly and time-consuming maintenance of the electrolyser module.
In an attempt to solve the problems associated with such degassing chambers, some electrolyser modules have been designed so that the degassing chambers are integrated with the cells. In some such designs, each electrolysis chamber of a cell stack is in direct communication with a degassing chamber through a first channel through which gas-liquid mixture enters the degassing chamber. Gas evolves from the gas-liquid mixture and the degassed liquid exits the degassing chambers through a second channel which is in direct communication with the appropriate electrolysis chamber and is situated below the first channel. For example, European patent application no. 1194716 (which is incorporated herein by reference) describes cell plates (referred to as holding frames) which define openings for an electrolysis chamber and openings for one or more degassing chambers where each cell plate is in direct communication with an associated degassing chamber. It has now been determined that integrated electrolyser modules of this design do not provide optimal conditions for removal of entrained gas from the liquid. In addition, such designs do not provide optimal conditions for increasing overall current efficiency of the electrolysis reactions and do not allow for effective differential pressure control between the anodic and cathodic electrolysis chambers. There is, therefore, a need for an electrolyser module which improves upon problems with the prior designs as described above.